Laser-based optical trapping has been used to show that optical forces are capable of displacing and levitating micron-sized dielectric particles in varied materials which thereafter resulted in development of the single-beam gradient force optical trap. Strategies to dynamically affect the steering of electromagnetic beams using optical trappings to enable varied forces on trapped objects in real time were also sought. As a result, certain of those developed strategies included scanning mirrors and acousto-optic deflectors (AODs), for example.
An acousto-optic deflector (AOD) consists of a transparent crystal inside which an optical diffraction grating is generated in relation to the density changes associated with an ultrasound acoustic traveling wave. The grating period may be typically determined by the crystal's acoustic wave wavelength and the first-order diffracted light that is deflected through an angle in relation to the acoustic frequency, via Δθ=λf/v (where λ is the optical wavelength, v is the velocity of the acoustic wave, and f is the frequency of the acoustic wave). The relation of f/v is the inverse of the ultrasound wavelength. An AOD may be able to control the trap position via deflection as well as the stiffness via light level.
As used herein, AODs may generally be described as providing a technique for altering a path of a beam of light that often involves propagating sound waves through a solid material. As sound waves propagate through a solid material, certain properties, such as a refractive index or lattice structure of the solid material, may be altered. In this manner, a light beam incident on a solid medium may be diffracted by a portion of a crystal lattice as it propagates through a crystal. Additionally, AODs may be switched quickly (<1 ms) and may be used in conjunction with focusing optics, such that they may scan a focused spot of light across a surface of a specimen for purposes inclusive of but not limited to inspection of a semiconductor wafer, for example.
FIG. 1 is an illustrative schematic example of a tunable AOD filter 100. The tunable AOD 100 has an acousto-optic medium 110, a transducer 120, an oscillator 130 or other input signal generator, and an absorber 140. It is known to use various crystals such as tellurium dioxide (TeO2) for instance as a medium in a tunable AOD. Incident light 150 is diffracted in relation to an optical λ tuning resulting from a frequency that creates an acoustic wave in the direction at 155. The angle of deflection creates a diffracted or tuned light beam 160. Tunable AODs are therefore configurable by adjusting their characteristics in relation to frequency, wavelength, access time and beam dimensions, for instance.
An AOD may be coupled to transducer which may be configured to generate a drive signal. The drive signal may create a chirp packet which may propagate through the AOD. The chirp packet requires a finite time to form, determined by the desired length of the chirp packet and the acoustic velocity in the AOD.
AODs also have widespread applications in the field of laser microfabrication and they are normally used for intensity modulation and laser beam steering. For example, an acousto-optic modulator (AOM) is used for the optical disk recording process to modulate the intensity of the writing beam based on the video or audio signal to be recorded. AODs are also commonly used in laser direct writing systems to provide a flexible, high-speed scanning ability with good precision and accuracy. Additionally, AODs have a variety of applications in light modulating, light deflecting and light filtering technologies.
However, the use of AODs for diffraction of electromagnetic radiation is understood thus far to be limited to only a single frequency grating imposed on the AOD. Additionally it is recognized that ultrafast laser pulses experience significant spatial and temporal dispersion while propagating through acousto optic materials, and the presence of temporal dispersion limits multiphoton excitation efficacy, and is particularly severe for acousto optic devices. As a result, the incoming electromagnetic radiation is not diffracted in relation to the point of incidence such that diffraction options and applications of AOD technology have heretofore been restrictive. What is needed is a system and method to overcome these limitations such as that including temporal and phase modulation of electromagnetic pulse trains, electromagnetic beams and full beams (collectively used herein as “beams,” “light beams,” “beamlets,” or “radiation”) so as to enable the advance or retardation of incoming electromagnetic radiation by introducing a time delay of the associated optical phase fronts and implementing a varying chirped wave on an AOD.